Thermionic electron tube circuit



Oct. 31, 1967 THERMIONIC ELECTRON TUBE CIRCUIT Filed Dec. 4, 1964 4Sheets-Sheet 1 FIG. I

INVENTOR TEN/VY D. LODE T. D. LODE 3,350,653

0a. 31, 1967 T. D. LODE 3,350,653

THERMIONIC ELECTRON TUBE CIRCUIT Filed Dec. 4. 1964 FIG. 2

I NVENTOR TENN) D. Z. 005

4 Sheets-Sheet 2 Oct. 31, 1967 T. D. LODE 3,350,653

THERMIONIC ELECTRON TUBE CIRCUIT Filed Dec. 4. 1964 4 Sheets-Sheet 4INVENTOR TENNY 0. L005 United States Patent Ofiice 3,350,653THERIVIIONI'C ELECTRQN TUBE CmCUIT Tenny D. Lode, Madison, Wis, assignorto Rosemount Engineering Company, Minneapolis, Minn., a corporation ofMinnesota Filed Dec. 4, 1964, Ser. No. 415,917 2 Claims. (Cl. 328270)This invention relates to thermionic electron tubes. More particularly,it relates to the automatic regulation or control of the heater power soas to increase the usefuel life of a thenrnionic electron tube.

Thermionic electron tubes, in spite of their advantages and widespreaduse, have been among the less reliable electronic components. Aparticular reliability problem is that thermionic tubes are subject notonly to gradual deterioration, which allows preventive maintenance andreplacement before complete failure. They are also subject to sudden andunexpected failures due to such causes as heater burn-outs and internalshort circuits. Such sudden and unexpected tube failures have remained amajor source of unreliability in electronic equipment incorporatingthermionic electron tubes.

The limited life and unreliability of thermionic electron tubes may beattributed largely to the high internal temperatures which are normallyrequired for their operation. The desirability of lower operatingtemperatures is well recognized. Various studies have indicated that theprobable life may be increased by a factor of two or more by a reductionof heater voltage of approximately 5 percent. Unfortunately, it is notpractical to simply reduce the heater voltage to a low value. As anextreme example, it might be pointed out that reducing the heatervoltage to zero would greatly extend the life of the tube at the penaltyof its not performing a useful task. The heater of a thermionic electrontube is normally designed so as to maintain a cathode temperature whichwill allow emission of the maximum design cathode current after tubeaging and at the minimum heater voltage limit. The penalty which is paidto meet these requirements is a shorter probable life than if thecathode were operated at a temperature just suflicient for the currentactually required in the specific application and without a reserve foraging and heater voltage fluctuations.

An object of the present invention is to automatically regulate theheater power so as to maintain the cathode of an electron tube atatemperature just sufficient to provide the required emission current ata particular time. A further object is to increase the probable life ofan electron tube and its associated equipment by reducing its averageoperating temperature.

In a particular form of the present invention, an electron tube isconnected in a class A voltage amplifier circuit arranged to amplifyalternating voltage signals. The plate current from the electron tube isfiltered or smoothed and passed through the control winding of amagnetic amplifier. The output of the magnetic amplifier is connected tothe electron tube heater. In operation, the magnetic amplifier will varythe heater voltage so as to maintain a desired average plate current.The required heater voltage will usually be significantly less than thenominal rated heater voltage of the electron tube. The electron tubeoperating temperatures will be correspondingly lower. Hence, thedeteriorating effects of high temperatures and temperature cycling uponthe electron tube and its surroundings are greatly reduced.

In the drawings:

FIGURE 1 is a schematic illustration of a first form of the invention,showing a class A amplifier circuit arranged for the amplification ofalternating voltage signals;

FIGURE 2 is a schematic illustration of a second form of the invention,showing a circuit arranged for the am- 3,359,653 Patented Get. 31, 1967plification of both unidirectional and alternating voltage signals;

FIGURE 3 is a schematic illustration of a third form of the invention,showing its application to the control of the average brightness of acathode ray tube presentation; and

FIGURE 4 is a schematic illustration of a fourth form of the invention,showing its application to the control of the peak brightness of acathode ray tube presentation.

In FIGURE 1, the cathode of electron tube 11 connects through resistor12 to ground 14. Capacitor 13 is connected in parallel with resistor 12,and serves as an alternating current bypass. The control grid ofelectron tube 11 connects via line 15 and capacitor 16 to input terminal17. Line 15 connects via resistor 18 to ground 19. The anode of electrontube 11 connects via capacitor 20 to output terminal 21. The anode ofelectron tube 11 also connects via resistor 22 to line 23. Line 23connects to a first side of resistor 24. Magnetic amplifier 25 includescontrol winding 26 and output Winding 27, both wound upon magnetic core28. The second side of resistor 24 connects to a first side of controlwinding 26, and the'second side of control winding 26 connects toterminal 29. Capacitor 30 is connected between line 23 and ground 31,and serves as a filter capacitor to isolate the electron tube circuitfrom alternating voltages developed across winding 26. Terminal 32connects to a first side of output winding 27, whose second sideconnects to the anode of diode 33. The cathode of diode 33 connects toline 34. Capacitor 35 is connected between line 34 and ground 35. Line37 connects from line 34 to a first side of the heater of electron tube11. The second side of said heater connects to ground 38.

The circuit of FIGURE 1 is generally that of a conventional thermionicelectron tube alternating voltage amplifier, except for the presence ofmagnetic amplifier 25 and the circuit elements associated therewith. Inoperation, a source of positive voltage is connected between terminal 29and ground to provide power for the plate circuit of electron tube 11. Asource of alternating voltage is connected between terminal 32 andground to provide power for the heater circuit of electron tube 11. Asource of alternating voltage signals which are to be amplified isconnected between input terminal 17 and ground. Terminal 21 serves asthe signal output terminal for the circuit, and output signals may bemeasured between terminal 21 and ground.

Magnetic amplifier 25 of FIGURE 1 is of the type 'known as a flux resetmagnetic amplifier. When the circuit of FIGURE 1 is first energized,electron tube 11 will be cold. Except for the transient charging currentof capacitors 20 and 30, essentially no current will flow throughcontrol winding 26 of magnetic amplifier 25. With no current flowingthrough control winding 26, output winding 27 will offer littleimpedance to the flow of current from AC terminal-32 through controlwinding 27, diode 33, into filter capacitor 35 and the heater ofelectron tube 11. Thus, the nominal full voltage will be applied to theheater of electron tube 11. As electron tube 11 warms up, current willbegin to flow in its plate circuit and through control winding 26. Asthe current through control winding 26 increases, the rectified outputof magnetic amplifier 25' and the heater power supplied to electron tube11 will be reduced.

In a properly designed circuit, the plate current and heater voltagewill stabilize at equilibrium values. At equilibrium, the plate currentthrough control winding 26 will generate a magnetic amplifier output, tothe heater of electron tube 11, just sufiicient to maintain the desiredaverage electron tube plate current. If the electron tube plate currentincreases, the heater voltage will be reduced so as to decrease theplate current to its nominal value. If the plate current decreases, theheater voltage will be increased so as to return the plate current toits nominal value. This regulation also provides a stabilization of theheater voltage with respect to variations of the AC power line voltage.Cathode resistor 12 maintains a small negative grid bias so that thecathode emission and plate current of electron tube 11 are partiallylimited by the negative potential of the grid with respect to thecathode, as well as being limited by the cathode temperature. A smallalternating voltage signal applied to the control grid will causecorresponding fluctuations of the instantaneous plate current ofelectron tube 11. A corresponding output signal will appear across plateresistor 22 and at output terminal 21. In class A-1 operation, in whichthe tube is not driven to cut off and in which the grid is at all timesnegative with respect to the cathode, the instantaneous variations ofthe plate circuit current caused by the alternating signal Will notsignificantly change the average current through control winding 26.Hence, these variations will not significantly affect the electricalpower input to the heater of electron tube 11 or the cathodetemperature. Thus, the circuit of FIGURE 1 may be used as an alternatingvoltage amplifier or control circuit. In a particular experimentalmodel, constructed along the lines of the circuit of FIG- URE 1, themagnetic amplifier employed a torroidal core wound from a .002 inchthick tape of a nickel-iron alloy known under the trade name ofDeltamax. Such cores have a rectangular hysteresis loop and areparticularly suitable for use in magnetic amplifiers. The controlwinding was wound with a suitable number of turns so that an electrontube plate current of the order of .001 ampere, the desired nominalplate circuit current, would generate a core magnetizing forceapproximately equal to the coercive force of the core material. Theoutput winding was wound with a number of turns such that it wouldinductively support a 6.3 volt 60 c.p.s. sinusoidal waveform, with coremagnetic flux excursions of approximately /2 to /3 of the saturationvalue. The B+ voltage at a terminal corresponding to terminal 29 ofFIGURE 1 was of the order of 250 volts. 6.3 volt AC power, derived froma secondary winding of a transformer connected to a 60 c.p.s. AC powerline, was connected between ground and a terminal corresponding toterminal 3.2. Resistors 12, 18, 22, and 24 were 1000 ohms, 470,000 ohm,100,000 ohms and 20,000 ohms respectively. Capacitors 13, 16, 20, 30 and35 were 50 microfarads, .05 microfarad, .05 microfarad, 8 microfarads,and 1000 microfarads respectively. Capacitors 13, 30 and 35 wereelectrolytic types, and capacitors 16 and 20 were of the paperdielectric type. Diode 33 was a conventional semiconductor junctionrectifier. The electron tube was a type 6AB4, a miniature triode with amedium amplification factor of approximately 20. The 6AB4 iselectrically similar to each of the two triode units in commonly used12AU7 dual triode, and is rated at a nominal heater voltage of 6.3. Theequilibrium plate circuit current of the experimental model wasapproximately .001 ampere, with a 1 volt negative grid bias due to thevoltage drop across resistor 12. The circuit functioned as a voltageamplifier in the manner described above.

When first connected and turned on, the equilibrium heater voltage forthe .001 ampere plate current was 3.4 volts. During the first 100 hoursof operation, the equilibrium heater voltage rose slowly to 3.6 volts.It then decreased slowly to 3.2 volts after 300 hours of operation. Fora subsequent year and a half of continuous operation, the equilibriumheater voltage remained at 3.2 volts. The tube continued to function asan amplifier and displayed no ill effects from continued operation at alow heater voltage. It is estimated that the cathode temperature Wasseveral hundred degrees cooler than when operated at the nominal 6.3heater voltage. The tube envelope remained relatively cool to the touch.

FIGURE 2 illustrates a second form of the invention which may be used toamplify or control unidirectional or low frequency signals. Electrontube 41 of FIGURE 2 is a dual control pentode in which the third grid isused to divide the cathode current between the second or screen grid andthe plate. Such tubes are used as variable gain amplifiers, and asmultiple input gates or switches in vacuum tube digital computing andcontrol circuits. The cathode of electron tube 41 connects throughresistor 42 to ground 44. Capacitor 43 is connected across resistor 42,and serves as an alternating current bypass. The anode of electron tube41 connects to output terminal 45 and to a first side of resistor 46.The second side of resistor 46 connects to line 47. The second or screengrid of electron tube 41 connects via line 48 to line 47. Magneticamplifier 49 includes control winding 50 and output winding 51, bothwound upon a magnetic core 52. Resistor 53 connects from line 47 to afirst side of control winding 50. The second side of control winding 50connects via line 54 to positive voltage supply terminal 55. Variableresistor 56 is connected between lines 47 and 54. Capacitor 57 isconnected from line 47 to ground 58. AC power input terminal 59 isconnected to a first side of output winding 51. The second side ofoutput winding 51 connects to the anode of diode 60, the cathode ofwhich connects to line 61. Capacitor 62 connects from line 61 to ground63. Line 64 connects from line 61 to a first side of the heater ofelectron tube 41. The second side of said heater connects to ground 65.The third or suppressor grid of electron tube 41 connects to inputterminal 66. The first or control grid of electron tube 41 connectsthrough capacitor 67 to input terminal 68. This first grid is alsoconnected via resistor 69 to ground 70.

The circuit of FIGURE 2 generally resembles the circuit of FIGURE 1,except for the substitution of three grid electron tube 41 and theaddition of variable resistor 56 as an adjustable current bypass aroundcontrol winding 50.

In the circuit of FIGURE 1, the average plate current is controlled bymagnetic amplifier 25. Hence, the circuit of FIGURE 1 is usefulprimarily to amplify and/ or control signals of frequencies which arehigh with respect to the response time of magnetic amplifier 25 and theheater and cathode of electron tube 11. Electron tube 41 of FIGURE 2 isa dual control pentode, in which the third grid is used to dividecathode current between the second or screen grid and the plate. Sincethe average potential of the first grid of electron tube 41 will be nearground, the cathode current of electron tube 41 will be brought to anequilibrium value in the manner described for the circuit of FIGURE 1.If grid #3, connected to terminal 66, is strongly negative with respectto the cathode, little current will be received by the anode of electrontube 41. The majority of the cathode current will be collected by thescreen grid and will flow through line 48. If a positive potential isapplied to terminal 66 and grid #3, a greater fraction of the cathodecurrent will be collected by the plate and a lesser fraction collectedby the screen grid. Since both screen and plate currents pass throughthe control winding circuit of magnetic amplifier 49, the control ofplate current by the #3 grid of electron tube 41 does not significantlyvary the total cathode current as seen by magnetic amplifier 49. Hence,low frequency or DC signals may be amplified and/ or controlled.Variable resistor 56 is an adjustable current bypass around controlwinding 50. This adjustable bypass is one means of adjusting thesensitivity of magnetic amplifier 49 and, hence, the equilibrium platecurrent of electron tube 41.

If desired, alternating voltage signals may be applied to input terminal68. The output voltage at terminal 45 will then be simultaneouslyaifected by signals applied to the first and third grids of electrontube 41.

FIGURE 3 illustrates a third form of the invention, showing itsapplication to the control of the average brightness of a cathode raytube presentation. In FIG- URE 3, the cathode of cathode ray tube 81connects via resistor 82 to ground 84. Capacitor 83 is connected acrossresistor 82. The beam current collection electrode of cathode ray tube81 connects to a first side of resistor 85 which connects in turn toline 86. Magnetic amplifier 87 includes first control winding 88, outputwinding 89 and second control winding 90, all wound upon magnetic core91. Resistor 92 connects from line 86 to a first side of control winding88. The second side of winding 88 connects to terminal 93. Capacitor 94connects from line 86 to ground 95. AC input terminal 96 connects to afirst side of output winding 89, which connects in turn to the anode ofdiode 97. The cathode of diode 97 connects to line 98, which connectsthrough capacitor 99 to ground 100. Line 98 connects through line 101 toa first side of the heater of cathode ray tube 81. The second side ofsaid heater connects to ground 102. Input terminal 103 connects via line104 to a control grid of cathode ray tube 81. Line 104 also connectsthrough resistor 105 to line 106. Line 106 connects through capacitor109 to ground 110, and through resistor 107 to a first side of winding90. The second side of winding 90 connects to ground 108.

The operation of the circuit of FIGURE 3 generally resembles theoperation of the circuits of FIGURES 1 and 2. The major differences arethe use of the invention for the control of the beam current of acathode ray tube, and the addition of second control winding 90 tomagnetic amplifier 87. A positive high voltage source is connectedbetween terminal 93 and ground to provide the high voltage for theoperation of cathode ray tube 81. An alternating voltage source isconnected between terminal 96 and ground to provide power for the heaterof cathode ray tube 81. A signal source, providing an electrical signalin accordance with the desired beam current and spot brightness, isconnected between terminal 103 and ground. The beam current from cathoderay tube 81 is passed through control winding 88 of magnetic amplifier87. Magnetic amplifier 87 controls the heater voltage applied to cathoderay tube 81 so as to maintain the desired average beam current and,hence, the desired average brightness.

In many instances, it will be desirable to vary the average brightnessof a cathode ray display in accordance with a received signal. In thecircuit of FIGURE 3, the received signal is applied to a control grid ofcathode ray tube 81 so as to vary the instantaneous beam current in thenormal manner. In addition, the signal from input terminal 103 isfiltered and applied to a second control winding of magnetic amplifier87. This will cause the equilibrium beam current and average displaybrightness to vary in accordance with the average received signal.

FIGURE 4 illustrates a fourth form of the invention, showing itsapplication to the control of the peak beam current and displaybrightness of a cathode ray tube. In FIGURE 4, the cathode of cathoderay tube 121 connects via resistor 122 to ground 124. Capacitor 123 isconnected across resistor 122. The beam current collection electrode oftube 121 connects via line 125 to a first side of variable resistor 126.The second side of variable resistor 126 connects to line 127, whichconnects to terminal 128. Line 125 connects to the cathode of diode 129,the anode of which connects to line 130. Capacitor 1-31 and resistor 132are connected in parallel between lines 127 and 130. Line 130 connectsto the base of transistor 13-3. The emitter of transistor 133 connectsto a first side of resistor 134, and through capacitor 135 to line 127.Magnetic amplifier 136 includes control winding 137 and output winding138, both wound upon magnetic core 139. The negative terminal of battery140 connects to the collector of transistor 133, and the positiveterminal to line 127. Terminal 141 connects to a first side of outputwinding 138, the second side of which connects to the anode of diode142. The cathode of diode 142 connects to line 143, which connects inturn through capacitor 144 to ground 145. Line 146 connects from line143 to a first 6 v side of the heater o f cathode ray tube 121. Thesecond side of said'heater is connected to ground 147.

The operation of the circuit of FIGURE 4 generally resembles theoperation of the previously described circuits. The major difference inFIGURE 4 is the addition of circuitry between the beam collectionelectrode of cathode ray tube 121 and the control winding of magneticamplifier 136. This additional circuitry causes magnetic amplifier 136to respond to the peak beam current, rather than the average beamcurrent. A positive high voltage source is connected from terminal 128to ground to furnish the high voltage for the operation of cathode raytube 121. A source of alternating voltage is connected between terminal141 and ground to provide power for the heater of tube 121. Anelectrical signal to vary the cathode ray tube beam current and spotbrightness as desired is connected between terminal 148 and ground.

Variable resistor 126, diode 129, capacitor 131 and resistor 132 form apeak holding rectifier circuit in which the stored voltage acrosscapacitor 131 corresponds to the peak current through line 125.Transistor 133 is con nected as an emitter follower to couple thecircuit of control winding 137 to capacitor 131 without drawingexcessive current. For convenience, the collector supply voltage totransistor 133 is shown as battery 140. In practice, it may be moredesirable to use a non-battery power supply for this circuit. With theaddition of this circuitry between line and control winding 137,magnetic amplifier 136 will control the heater voltage of cathode raytube 121 so as to maintain a desired peak beam current and peak spotbrightness. Varying the magnitude of variable resistor 126 will changethe sensitivity of the magnetic amplifier control circuit and theresult-ant peak beam current of cathode ray tube 121.

The circuit of FIGURE 4 may be particularly useful for radar displayapplications in which it is desired to maintain a constant peakbrightness, in spite of changes in the average brightness due to factorssuch as variations in the size and number of displayed targets.

The drawings and the preceding description have shown the control of thecathode temperature and cathode current of 1 and 3 grid thermionicelectron tubes, and of grid controlled thermionic cathode ray tubes.Similar techniques may be employed to reduce the operating temperatureand increase the reliability and probable life of other forms ofthermionic electron tubes such as klystrons, traveling wave tubes, andother thermionic electron devices in which an electron beam may becontrolled both by varying the cathode heater power and by usingadditional electrostatic and/ or electromagnetic means.

The preceding description has implied the use of indirectly heatedcathodes, electrically separate from the cathode heater element. Similartechniques could be used with directly heated cathodes, such as filamenttype cathodes. The preceding description has shown the use of a magneticamplifier to control the heater voltage or power. Other forms ofamplifying and controlling means, for example, transistor amplifiers,may be similarly used with similar results.

What is claimed is:

1. An electron tube circuit comprising an electron tube having athermionic electron emitting cathode, an anode and a control grid,heating means for varying the cathode temperature, means coupled to theheating means for controlling the magnitude of the heating therebycontrolling the cathode temperature, power terminals for connecting adirect voltage supply in series with the anode and cathode so that acurrent may flow from anode to cathode, signal terminals for applying avoltage signal to the control grid, first means responsive to the anodecurrent effecting a first control of the magnitude of the heating, andsecond means responsive to the voltage at the signal terminals effectinga second control of the magnitude of the heating, whereby thetemperature of the cathode is controlled in part by the anode currentand in part by the voltage at the signal terminals.

2. An electron tube circuit for controlling the average brightness of acathode ray tube presentation in response to a voltage signalcomprising, a cathode ray tube having a cathode heater, a cathode, ananode and a control grid, a magnetic amplifier having a first controlwinding connected to the anode, a second control winding connected tothe grid and an output winding connected to the heater, means forpassing a current through the output winding and the heater whereby theoutput winding current is effective in controlling the cathodetemperature, means for passing a beam current through the first controlwinding and between the cathode and anode whereby the beam currentaffects the output winding current to cause the cathode temperature tovary inversely with beam current, and signal terminals for applying asignal voltage to the grid and a signal current through the secondcontrol winding whereby the signal current affects the output windingcurrent to cause the cathode temperature to vary with signal current.

References Cited UNITED STATES PATENTS 2,236,195 3/1941 McKesson 3l51062,834,883 5/1958 Lukoft 328151 2,940,010 6/1960 Kenny 315l06 DAVID J.GALVIN, Primary Examiner.

1. AN ELECTRON TUBE CIRCUIT COMPRISING AN ELECTRON TUBE HAVING ATHERMIONIC ELECTRON EMITTING CATHODE, AN ANODE AND A CONTROL GRID,HEATING MEANS FOR VARYING THE CATHODE TEMPERATURE, MEANS COUPLED TO THEHEATING MEANS FOR CONTROLLING THE MAGNITUDE OF THE HEATING THEREBYCONTROLLING THE CATHODE TEMPERATURE, POWER TERMINALS FOR CONNECTING ADIRECT VOLTAGE SUPPLY IN SERIES WITH THE ANODE AND CATHODE SO THAT ACURRENT MAY FLOW FROM ANODE TO CATHODE, SIGNAL TERMINALS FOR APPLYING AVOLTAGE SIGNAL TO THE CONTROL GRID, FIRST MEANS RESPONSIVE TO THE ANODECURRENT EFFECTING A FIRST CONTROL OF THE MAGNITUDE OF THE HEATING, ANDSECOND MEANS RESPONSIVE TO THE VOLTAGE AT THE SIGNAL TERMINALS EFFECTINGA SECOND CONTROL OF THE MAGNITUDE OF THE HEATING, WHEREBY THETEMPERATURE OF THE CATHODE IS CONTROLLED IN PART BY THE ANODE CURRENTAND IN PART BY THE VOLTAGE AT THE SIGNAL TERMINALS.